Navigate to the page you wish to modify

Click on Edit.

Click on the  sign below the title box.  This prompt will also appear between any existing web parts. 

From the web part gallery select Text.

The text web part will appear. Start typing in the Add your text here box.

Font style, bold, italics, underline, lists, Indenting, alignment, hperlinks and image insertion can be found on the Formatting toolbar

For more options click on the ellipsis (...) to bring up the Property pane.

The Text and table formatting pane adds many more formatting options including text strike through, superscript and subscript, indenting, font size and colour, Right to left/Left to Right paragraphs and alignment can be found in the Paragraphs section size,colour.

You can add a table to the text box by selecting the Insert Table button. 

When you add a table a specific formatting toolbar will also appear.

If you want to clear all formatting, click the button towards the top of the pane.

When you have finished, close the pane by clicking on the cross in the top right.